So I made an app focused (hehe.) on ease of switching between noise intensity. There's a long explanation why it's important if you want one (https://incentiveassemblage.substack.com/p/why-is-nobody-ser...), but if you're the kind of nerd who knows about 'flow state' and 'Yerkes-Dodson law', you pretty much got everything covered. The core point is: if intensity of your main activity is changing, your background noise should too, so that you maintain the same total level of arousal. Most likely double so for ADD people.
My current solution is to use media buttons (forward/backward) to control intensity. It took much more work than you'd expect to get media interface to do this without breaking currently playing sounds. In general anything that isn't 'Play this sound' works much worse than one would anticipate with how prevalent media is on the internet - I see ambiphone does the same thing with playing sounds separately, but you saved yourself from managing media interface, so I can't quite tell how much pain exactly you have experienced with this.